This isn't unique to me, but I'll have a go:
I dislike playing with munchkins who grab everything that isn't nailed down. For example, stealing blankets from the houses of friendly NPCs.
I dislike playing with PCs who keep secrets from the party. E.g. a munchkin who stole silver daggers from the friendly NPCs quietly kept mum about those silver daggers when we were facing wererats. All the players knew he had those daggers of course, and so in character we were like, "Does anyone have any silver weapons? No? Okay, I guess we'll have to do something else then..." He probably had some kind of in-character motivation but that didn't make it any less contemptible, that just makes him contemptible on purpose--either way I don't want that guy in the party.
As a player, I also dislike playing with PCs or players who make dreadfully bad tactical decisions. I don't mean character-building decisions like making a wizard with Str 16 and Int 12--that's weird, but it's part of the metagame so it can't be stupid. But a multiclassed Wizard 9/Cleric 9 who consistently blows his 9th level spell slots casting Chromatic Orb and Mass Healing Word drives me nuts.